Solid state light sources, such as light emitting diodes (LEDs), are widely used in lighting products for commercial and personal use, including, for example, indoor and outdoor lighting applications and backlighting displays for monitors and televisions. Incandescent and fluorescent bulbs and tubes have long been the standard in the lighting industry. Incandescent and fluorescent bulbs and tubes can be inefficient in the use of energy, can have short lifespans, and/or can cause disposal problems. For example, compact fluorescent lamps (CFL), while having longer life spans than incandescent lamps, have a relatively short lifespan. Due to the chemicals, for example, Mercury, used inside such lamps, these lamps cannot be disposed of after use in the normal course of garbage disposal. Disposal of such CFL lamps for large facilities is expense and can be time consuming due to the procedures that should be followed.
LEDs can be used in the design of compact, thin, energy-saving products having longer lifetimes than conventional lighting products on the market. Products using LEDs require less power to meet the brightness specifications for a given lighting application, thereby significantly reducing energy consumption and the need for active cooling systems. A current trend in packaging LEDs is the use of thinner molded packages for fitting into thin, possibly flat, panel display systems. Thinner packages can, for example, have increased cavity angles to assist in exceeding or maintaining brightness specifications. As cavity angles increase, package material can incompletely mold about package components. For example, package material can incompletely mold about portions of a leadframe. This can lead to gaps, voids, incomplete resin filling, and low adhesion between components within a given package.
In recent years, there have been dramatic improvements in light emitting diode (LED) technology such that LEDs of increased brightness and color fidelity have been introduced. LED efficiencies are set to exceed those of fluorescent tubes, with dimming and controllable color rendering readily achievable. Multichip LED lamps can be mounted and used in fluorescent fittings, with ballast replaced by driver electronics. Spatial distribution, intensity and spectrum of light output from LED lamps in fluorescent fittings can be comparable to those produced by a fluorescent tube with the same or less power input. LED lamps in such fluorescent fittings, however, can be relatively expensive to manufacture. Smaller LEDs are desirable in such applications. Also, the LEDs can also create heat levels that, if they became excessive and/or the heat is not properly dissipated, can lead to LED and/or circuitry failure.
Additionally, due to these improved LEDs and improved image processing technology, large format, full color LED video screens have become available and are now in common use. Large format LED displays typically comprise a combination of individual LED panels providing image resolutions determined by the distance between adjacent pixels or “pixel pitch.”
Outdoor displays, which are intended for viewing from greater distances, have relatively large pixel pitches and usually comprise discrete LED arrays. In the discrete LED arrays, a cluster of individually mounted red, green, and blue LEDs are driven to form what appears to the viewer as a full color pixel. On the other hand, indoor screens, which require shorter pixel pitches such as 3 mm or less, typically comprise panels carrying red, green, and blue LEDs mounted on a single electronic package such as a surface mount device (SMD) package. Each SMD usually defines a pixel. The relatively small SMDs are attached to a driver printed circuit board (PCB) that controls the output of each SMD.
Although both indoor and outdoor displays are viewable across a substantial range of off-axis angles, there is often a perceptible loss of color fidelity with increasing viewing angle. Additionally, the material of each LED package and/or the material used to mount each of the LEDs may have reflective characteristics, which can further decrease color fidelity by creating unwanted light reflection and/or glare.
It is well-known that SMDs and many other types of electronic packages, whether containing integrated circuits or discrete components such as diodes or power transistors, dissipate sufficient heat to require thermal management. Also, excessive heat may cause LEDs failures. Thus, one of the considerations for designing an LED system is effective thermal management. One of the objectives of effective thermal management in the design of electronic packaging is to maintain the operating temperature of the LEDs and other active circuit components at an appropriately low level to prevent premature component failure. Various cooling strategies including conduction heat transfer are in common use. One conventional way of implementing conduction heat transfer for dissipating heat in an electronic package is to allow the heat to conduct away along the leads of the device. However, the leads often do not have sufficient mass or exposed surface area to provide effective heat dissipation. For example, high intensity LEDs that emit light principally in the visible part of the electromagnetic spectrum can generate a significant amount of heat that is difficult to dissipate using such conventional techniques.
The designing objectives of increasing the view angle, maintaining a relatively low operating temperature, and decreasing the size of an LED package are to some extent competitive with each other. There is a need therefore to develop an LED package that addresses all these designing objectives with lower cost.